This grant describes biochemical experiments designed to characterize bacteriophage repressor and antirepressor proteins and to understand the mechanisms by which they regulate gene expression. The binding of the phage lambda repressor to its operator will be studied by crosslinking, chemical modification, and mutant repressor studies. We envision that these combined studies will identify specific functional groups of the repessor protein that are involved in operator recognition. We will also study the thermodynamics and kinetics of repressor binding to DNA in order to under the types of forces which stabilize repressor-operator binding and the mechanism by which specific sequence recognition occurs. We will isolate and sequence the Salmonella phage p22 mnt repressor and its operator. Thus far these control elements have been studied only by genetics. We will also isolate and characterize the p22 antirepressor protein whose synthesis is regulated by mnt. The antirepressor inactivates many phage repressors, including the lambda repressor, and we will probe the interaction between ant and lambda repressor. These experiments will determine which regions of the two proteins are involved directly in the interaction and should explain how antirepressor prevents lambda repressor from binding to its operator.